I have nothing against someone wanting to eat ass, but seriously that's just saying you want to digest shit out of someone's asshole, with a presumably metal eating utensil going into that person's asshole.

Errr, to be precise, if the quote is "eat her ass with a spoon" it would describe the attempted procedure of trying to carve the meat out of another persons built-in sitting cushion for personal gustatoric pleasures, making an ass out of yourself attempting it as well by realising that the spoon wont even cut through the skin, let alone be able to carve the flesh out of it.

I think you misinterpret "eating an ass with a spoon" as a figure of speech impliying you "dig" said ass, with "eating OUT OF an ass with a spoon", the (slightly disturbing) procedure you described